The improvement of image quality by the elevation of an automatic focusing function is increasingly required along with the popularization of digital cameras in recent years. The conventional AF apparatuses generally use the contrast detection method for focusing in which a focus lens position where the maximum value of the high frequency component in the luminance signal of an object image formed on an image-pickup element is obtained is determined as an in-focus position.
In addition, a camera is known, which has a plurality of focus detection areas in its image-taking screen and performs focusing for the closest-object-distance area of a plurality of focusable focus detection areas so that focusing on an object existing in an area other than the central area in the image-taking screen can be performed.
However, the inclination or center misalignment between the image-taking lens and the image-pickup element, or the inclination of lens units constituting the image-taking lens, which is caused by manufacturing error, deteriorates the accuracy of focus detection. Further, the automatic focusing on the basis of focus detection information with low accuracy causes a so-called one-side blur. The one-side blur means a state in which one side portion of the image-taking screen is in an in-focus state but the opposite side portion or the central portion thereof is in an out-of-focus state.
For solving the problems, a camera has been disclosed in Japanese Patent Laid-Open Application No. 2003-222787, in which the difference information between the object distances of the in-focus focus detection areas (hereinafter, it is referred to as an in-focus distance) is compared with a reference difference, and the focus control is changed according to whether the difference information is larger than the reference difference or not. According to Japanese Patent Laid-Open Application No. 2003-222787, to reduce the one-side blur, focusing on the central focus detection area is performed if the in-focus distances of the focus detection areas are the same.
However, the camera disclosed in Japanese Patent Laid-Open Application No. 2003-222787 has problems that it is difficult to obtain an in-focus state to an object in an area other than the central focus detection area, and it is difficult to obtain a best in-focus state to the same distance objects though a large blur does not occur.
For example, in a scene shown in FIG. 7, the focusing on a floor lamp S behind a table T located in the central area of the image-taking screen is performed if the in-focus distance difference between two persons P1 and P2 located on either side of the table T and the lamp S is less than a predetermined difference.